This invention relates to a control for energizing a heater provided to raise the temperature of lubricating oil of a compressor, and in particular to a control for selectively energizing the heater in response to a temperature indicative of the lubricating oil temperature.
It is well known that under certain conditions, some refrigerants and oil used as a lubricant for the compressor of a refrigeration unit are freely miscible. During normal operation of the refrigeration circuit, because of operating pressures and temperatures, the oil in the sump of the compressor, will be substantially free of refrigerant. However, on shutdown when the circuit reaches ambient temperature, and the pressure equalizes within the circuit, the refrigerant vapor and oil in the sump of the compressor will mix to form a substantially homogenous solution. This phenomenon becomes increasingly evident as the ambient temperature decreases.
Upon startup of the compressor, the oil sump which is usually a part of the crankcase of the compressor drops to suction pressure and the compressor mechanism may agitate the mixture of lubricating oil and refrigerant. The combination of the drop in suction pressure and possible mechanical agitation causes the refrigerant in solution to attempt to return to its vapor state. Since the refrigerant at shutdown is in a substantially homogenous solution, the flashing of admixed liquid refrigerant to vapor may carry therewith a substantial amount of the oil charge and may even result in the entire solution turning into a foam.
Foaming of the oil will materially increase the amount of oil carried over into the refrigerant discharge line. Foaming may become so severe that all of the oil is pumped out of the sump. Not only will this leave the compressor without lubrication, which may produce excessive bearing wear and bearing failure in a very short period of operation, but there is also the possibility that noncompressible slugs of liquid refrigerant and oil will enter the compressor's cylinders and cause serious damage to the compressor in the form of broken valves and pistons and bent or broken connecting rods and shafts.
To avoid the problem of crankcase oil dilution, heaters are generally employed. The heater may be an electrical resistance element. The resistance element may either be installed directly in the sump of the compressor, in direct contact with the oil, or may be wrapped around the outer surface of the compressor casing in heat transfer relation with the oil stored in the sump. The energization of the heater will maintain the lubricating oil at a satisfactory temperature above ambient temperature, for example 40.degree. to 60.degree. F above the ambient. At this temperature, only a small amount of refrigerant will be absorbed by the oil charge.
Heretofore, it has been the practice within the industry to either maintain the heater energized at all times regardless of the operation of the refrigeration unit or of the temperature of the ambient. Alternatively, it has been the practice to render the heater inoperable when the refrigeration unit is functioning and to energize the heater when the refrigeration unit has been shut down. In either case, operation of the heater, when the ambient temperature is above a predetermined level, for example 70.degree. F, is wasteful of energy. When the temperature of the ambient is relatively warm, only a relatively small amount of refrigerant will be absorbed by the lubricating oil during the time in which the refrigeration unit is inoperable. The minimal quantity of refrigerant that may be absorbed, will not cause damage to the compressor and thus may be tolerated. At a time when the conservation of energy is in the national interests, and when it has been increasingly desirable to decrease operating costs, it is evident that the continued operation of an electrical device, such as the aforedescribed heater, when such operation is not required, is extremely undesirable.